Germany's post-World War Two justice ministry was infested with ex-Nazis hellbent on protecting their former comrades, according to a new official study released Monday.

Fully 77 per cent of senior ministry officials in 1957 were former members of Adolf Hitler's Nazi party, a higher proportion even than during the 1933-45 Third Reich, the study found.

"We didn't expect the figure to be this high," said study co-author Christoph Safferling, who evaluated former ministry personnel files, speaking to the daily Sueddeutsche Zeitung.

At a time when a fierce struggle was underway about punishing Nazi crimes, the old comrades were reluctant to come under the scrutiny of young, unencumbered outsidersChristoph Safferling

The fascist old-boys network closed ranks, enabling its members to shield each other from justice, the study found - helping to explain why so few Nazi war criminals ever went to prison.

"The Nazi-era lawyers went on to cover up old injustice rather than to uncover it and thereby created new injustice," said Heiko Maas, Germany's justice minister, who presented the report Monday.

Though the report only deals with ministry bureaucrats, it roughly matches figures for the wider justice apparatus.

Historians have previously found that in the 1950s, more than 70 per cent of West Germany's top judges also had former Nazi connections.

Safferling said that "at a time when a fierce struggle was underway about punishing Nazi crimes, the old comrades were reluctant to come under the scrutiny of young, unencumbered outsiders".

While other government ministries were able to look to Germany's future, said Safferling, "justice always deals with the past".

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Those officials questioned about their roles in the Holocaust machinery usually claimed ignorance, said they had only followed orders, or argued that by staying in their posts they had prevented even worse crimes.

"You never find words of regret, only justifications," said Safferling.

The ministry largely failed to employ jurists who had been in exile during the war or had joined the anti-Nazi resistance, the Rosenburg project - named after the first ministry building in the former West German capital Bonn - found.

Between 1949 and the early 1970s, 90 of the 170 top ministry officials were former Nazi party members, and many had served as Holocaust-era judges who had handed down death sentences, said former justice minister Sabine Leutheusser-Schnarrenberger, who initiated the study in 2012.

The report casts new light on the early post-war era's collective amnesia about Nazi crimes in then West Germany.

Germans were then focused on rebuilding a country in ruins, and many remained in denial about past crimes, dismissing the 1945-49 Nuremberg trials as "victor's justice".

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The western allies occupying the country - the United States, Britain and France - were focussed on what they saw as the new Cold War threat of Soviet Russia, which controlled communist East Germany.

As a result of these and other factors, allied and later West German courts only convicted around 6,650 ex-Nazis, a tiny fraction of the party membership.

Few of these served long jail terms because judges required a high burden of proof, including evidence a defendant had acted on their own initiative or with unusual cruelty. Most suspects were regarded as accomplices and thus received lighter sentences.

A fundamental change only came after most war criminals were dead, with the case of Ukrainian-American John Demjanjuk, a former death camp guard who had become an auto plant worker in Ohio.

In 2011 a German court convicted him not for crimes he could be linked to personally, but on the basis that he had served as a cog in the vast Nazi killing machine, at the Sobibor camp in occupied Poland.